Everyday Aviation.

Aircraft Pressurization Beginner’s Guide

How and why are airplanes pressurized?

It’s easy to take flying for granted. We hop onboard a comfy airliner and fly high in the stratosphere without giving breathing a second thought. The aircraft’s pressurization system makes it possible. Here’s how the magic works…

Hypothetical experiment: If you place a scale in a vacuum chamber, fill a balloon with air, then compare its weight to that of an unfilled balloon and you’ll see that air has mass.

Earth’s atmosphere is about 300 miles thick. At sea level, our bodies are subjected to about 14.7 pounds of pressure from this tall column of air. I’ll bet you don’t even notice! For animals roaming the earth’s surface, a 14.7 psi atmosphere provides the perfect amount of oxygen.

As we climb in altitude, the amount of air pressure acting on us decreases rapidly. You notice the decrease when your ears pop while driving up a mountain or riding a fast elevator. Although the atmosphere is 300 miles thick, most of the air molecules are squashed down to within a few thousand feet of the earth’s surface.

Denver is fine. Going higher spells trouble.

As we climb higher, our lungs take in less oxygen molecules. Folks living in Denver, Colorado (5600 ft) are quite happy breathing the lower, 12 psi atmosphere. Climbing to a higher altitude, though, and the pressure drops really fast.

At 18,000 feet, the atmospheric pressure is down to 7.3 psi, about half the sea-level pressure. There just isn’t enough oxygen in a breath of air to adequately supply the brain. At this pressure, a healthy adult has only 20-30 minutes of useful consciousness.

Airliners fly between 30,000 and 43,000 feet. At those altitudes the atmosphere provides less than 4 psi of pressure. If you tried breathing air that “thin” your useful consciousness would be less than a minute (followed soon after by death).

To survive high altitudes, occupants of an aircraft need help breathing. The solution is to pump air into the airplane so the interior pressure is high enough to keep the humans happy.

Why bother with pressurization? Why not fly down low?

Airplanes can certainly fly below 10,000 feet where the atmospheric pressure is a comfy 10 psi or higher, but it has some drawbacks:

It’s tough to cross a 14,000 foot mountain range at 10,000 ft.

Most bad weather is at lower altitudes.

Turbofan engines are very inefficient down low.

Aircraft ground speeds are slower at lower altitudes.

If you want a fast, smooth ride in a fuel efficient airplane that can fly over a mountain range, we need to pressurize!

How does a pressurization system work?

The airplane body (fuselage) is a long tube capable of withstanding a fair amount of differential air pressure; think of it like a big plastic soda bottle. In theory, we could seal the bottle so, as the airplane climbs, the interior air pressure would stay the same. We can’t do that because it’s hard to perfectly seal a huge airplane fuselage. Even if we could, the passengers would quickly use up the available oxygen. And just imagine the smell inside a perfectly sealed tube on a long flight! Clearly, a big sealed soda bottle won’t work for us without some modification.

A fuselage is a bit like a soda bottle with a hole in the back.

To solve the problems, pressurization systems constantly pump fresh, outside air into the fuselage. To control the interior pressure, and allow old, stinky air to exit, there is a motorized door called an outflow valve located near the tail of the aircraft. It’s about the size of a briefcase and located on the side or bottom of the fuselage. Larger aircraft often have two outflow valves. The valves are automatically controlled by the aircraft’s pressurization system. If higher pressure is needed inside the cabin, the door closes. To reduce cabin pressure, the door slowly opens, allowing more air to escape. It’s one of the simplest systems on an aircraft.

The outflow valve on a Boeing 767-300. Photo by author.

One of the benefits of a pressurization system is the constant flow of clean, fresh air moving through the aircraft. The air inside the airplane is completely changed every two or three minutes making it far cleaner than the air in your home or office.

Pressurization systems are designed to keep the interior cabin pressure between 12 and 11 psi at cruise altitude. On a typical flight, as the aircraft climbs to 36,000 feet, the interior of the plane “climbs” to between 6000-8000 feet.

Exterior and interior altitude profile on a typical flight.

Why not keep the cabin at 14.7 psi to simulate sea-level pressure and maximize comfort? The aircraft must be designed to withstand differential pressure, that’s the difference between the air pressure inside and outside the aircraft. Exceeding the differential pressure limit is what makes a balloon pop when it’s over inflated. The greater the differential pressure, the stronger (and heavier) the airplane must be built. It’s possible to build an aircraft that can withstand sea-level pressure during cruise, but it would require a significant increase in strength and weight. A 12 psi cabin is a good tradeoff.

That’s just nasty!

Outflow Valve Trivia:

If you look at pictures of airliners taken prior to 1990, you might see brown stains around the outflow valve. The stains are from tobacco smoke. Airlines were thrilled when the industry banned smoking. Tar and nicotine gummed up valves, instruments, and sensors causing thousands of dollars a year in damage. Tobacco is really nasty stuff.

Where does pressurized air come from?

Electric Compressors
Old piston powered airliners, like the Boeing Stratocruiser, used electric air compressors to pump fresh, outside air into the cabin. This system worked well, but the compressors added a lot of weight to the aircraft.

Engine Bleed AirMost modern airliners use bleed air from the compressor section of the engines to pressurize the cabin. This very hot air must be cooled to a comfortable temperature before it’s directed into the cabin.

Electric Compressors (Again!)
The new Boeing 787 Dreamliner brings back the electric compressor. The 787’s electrical system powers compressors, just like on the old Stratocruiser. Advances in technology make this system far more efficient than it’s predecessor from the 1950’s.

What is bleed air?

A jet engine has three main sections: compressor, combustion, and turbine/exhaust. The compressor is at the front of the engine. A series of spinning blades draws in fresh, outside air. As the air is compressed, it becomes very hot. Remember high school physics? As a gas is compressed, its temperature rises. The hot, compressed air then enters the combustion chamber where it is mixed with fuel and burned. The expanded gasses continue through turbine blades which power the compressor blades before exiting the engine as thrust.

Bleed air is fresh, clean, hot air taken from the compressor section of the engine before it is mixed with fuel or exhaust gasses. Common uses for hot bleed air are wing and engine ice protection, cabin pressurization, engine starter motors, and air driven hydraulic pumps.

How do pilots control the pressurization?

Pressurization controls on a 757 & 767.

It’s really, really easy. The cabin altitude control panel on the 757 and 767 is super simple. During preflight checks, we turn the “LDG ALT” knob to display the altitude of the landing airport. That’s it! We don’t touch it for the remainder of the flight. The automatic mode takes care of the outflow valve for us.

The remaining indicators and knobs are for redundancy in case of a malfunction. There are two separate automatic modes. A manual mode allows us to adjust the position of the outflow valve should both auto systems fail. Pressurization systems work great and rarely cause any trouble.

Effects Of Flying In A Pressurized Cabin

The air inside an aircraft cabin is very low in humidity. On a long flight it’s important to drink plenty of water to stay hydrated. When the flight attendant offers you a bottle of water, drink it. You may not notice that you’re dehydrated.

Alcohol consumption: Dehydration increases the effects of alcohol on your body. To make matters worse, alcohol increases dehydration; it’s a double-whammy. If you choose to drink alcohol on a flight, be sure to drink plenty of water and have something to eat while enjoying your cocktail. Don’t be that guy. Drink extra-responsibly when flying.

Does this food taste bland? Yes! There’s a good chance your in-flight meal really does taste bland. The aircraft cabin’s low humidity and lower air pressure reduce your sense of taste and smell by as much as 30% according to a Lufthansa commissioned study. Airline food kitchens often add extra spices and flavoring to meals to compensate for your crippled taste buds!Special thanks to my Twitter friend (and fellow blogger) @Jen_Niffer for tipping me off to the Lufthansa study!

Further Reading About Pressurization:

AeroSavvy is written by Ken Hoke. Since 1984, Ken has loitered the skies in many vehicles, most notably the classic Douglas DC-8. He currently frustrates air traffic controllers in the US, Asia, and Europe as a Boeing 767 captain for a package express airline.
Ken can be reached here or any of these fine social media outlets:

Yes, I remember those days…I’ve had asthma all my life. Even though I was seated as far as possible from the “smoking section”, we were ALL breathing it. That’s probably responsible for at least some of the irreparable damage to my lungs and COPD which now hinder my travel. Thank goodness those days are over! Thanks for your clear and concise explanations!

Great post Ken! Informative but also quite entertaining. Loved the graphics — and I had no idea the 787 used electric motors to compress air for the pressurization system. I also didn’t know abut the tar stains on the outflow valve… but I should have. That feces-color staining is unmistakable. Truly disgusting.

Before we take off, we adjust the LDG ALT (Landing Altitude) knob so the display shows the elevation above sea level of the landing (destination) airport. When I took the photo in the article, we were in Shenzhen, China (near Hong Kong) getting ready to depart to Kuala Lumpur, the capital city of Malaysia. Kuala Lumpur’s airport elevation is 70 feet above sea level, so that’s what we set in the window.

As we begin our descent for landing, the pressurization system uses the LDG ALT information to make sure the air pressure inside the plane is the same as the outside pressure when the airplane touches down on the runway.

This is one of those worst case scenarios! Systems on every aircraft model are different, so my answer will be based on a “generic,” bleed air pressurized aircraft; we’ll call it the AeroSavvy AS-100.

In the event of all engines failing, we’ll lose our primary sources of pressurization. With no air being pumped into the cabin, the outflow valve will close in an attempt to maintain cabin pressure. Even though the valve is closed, the air still leaks out through various cracks and crevices causing the cabin pressure to slowly drop. It won’t be a rapid depressurization, but your ears will start popping.

IF the aircraft’s APU (auxiliary power unit) is running, it may provide enough bleed air to pressurize the cabin and keep the masks from dropping. The crew will likely turn on the APU as soon as they see the engines failing.

In the meantime, the crew will have the aircraft in a controlled, gradual descent as they run through the engine restart checklist. Once the engines are running again, normal pressurization will be restored.

The good news is that this has never happened on an AeroSavvy AS-100. 🙂
Sorry for the long answer.
Thanks for reading!

Thanks Ken
This article explains aircraft pressurisation so clearly. I have forwarded the link to my book group. Why? One of the members wondered why her smuggled boxes of eggs burst in her hold suitcase.

Were they raw eggs? Eggs have very little air inside of them which should make them fairly immune to the pressure changes in an airliner. If she checked her bag, it was likely the baggage crews that caused the eggs to break.

I recently flew on a 787 for the first time. I was hoping that the extra cabin pressure on a 787 would help my symptoms from sinus issues be less apparent. By midway of the long flight I had all the various pain issues that I associate with sinus problems while flying. This made me wonder if the 787 actually is flown at the advertised pressure you see in the literature or airlines can set the amount as they see fit? (I also realize that my discomfort might be from something else; dehydration or just sinus problems not related to the pressure.) – Hans

I’m sure the airline was operating the 787 pressurization exactly as it was designed to be used. It’s an automated system that works beautifully. Even though the system is an improvement over older systems, it still does not maintain a ground-level cabin pressure. While a typical airliner might maintain a 7000-8000 foot cabin altitude, the Dreamliner has a 6000-7000 foot cabin. While this is a big improvement, someone with sinus problems will likely still have problems in a 787.

I’m sorry about your discomfort. You might check with your doctor before flying again. He or she may be able to recommend something like a Benzedrex inhaler that will help relieve your discomfort.

I see an aircraft pressurization system advertised as 5.5 psi. Is there a chart or formula which would indicate the cabin pressure versus sea level pressure at 5.5? As I am reading various responses, I am guessing that it may be close to 13,000 ft MSL. Yet I have been unable to find a chart.

If the 5.5 psi reference you saw referred to differential pressure, it wouldn’t be nearly high enough to pressurize an airline cabin. Airliners typically run about 7-8 psi differential pressure at cruise (differential pressure is the pressure inside the cabin minus the pressure outside).

If you can provide a link to where you saw the information, perhaps I can decipher it for you.

The various versions of the Piper Malibu it has a maximum cabin pressure differential between 5.5 and 5.6; that is probably the plane he saw that data on. Because they fly at a much lower altitude the lower differential is not as much of an issue as in an airliner.

Great question, Ben!
The very first aircraft with pressurization was the Airco DH.9A. The British WWI bomber was modified in Dayton, Ohio to have a pressurized compartment. In 1921, this redesignated aircraft, the US D-9A, flew the first high altitude pressurized flight. I don’t know if any one person “invented” pressurization. The US D-9A work was done by the Aviation Section, U.S. Signal Corps and its successor the United States Army Air Service.

On the aircraft I fly, there are no detection or warning systems to let us know of contaminated bleed air. There are only a few things that can contaminate bleed air.

Occasionally, deicing fluid can get into the system; we have procedures that we follow to minimize this risk. The very small amounts that get in the system (usually through the APU inlet) are not hazardous. The large amount of fresh air flowing through the system quickly dissipates the residual vapor.

Sometimes vapor from engine lubricants can find themselves in the bleed air system as the engines are started. Again, the small amounts of vapor are quickly eliminated from the system by the huge quantity of fresh air being moved through the aircraft.

It’s possible that a temperature control valve can malfunction causing excessively hot air in the system. We’ll get an indication in the cockpit that this is happening and we will likely smell it. The crew has a checklist for this and the system responsible for the problem will quickly be shutdown. This is pretty rare and redundant systems assure that it’s not a big deal.

That’s about it for bleed air. Because of the way the system is designed, it is very difficult for the air supply to become compromised.

Great article, Ken. On a recent flight to TPA on an older 737-300, ears were popping as usual on the way up and down. Return trip on a 700, the pressurization was so smooth the thing I noticed was the very little bit of ear popping I had. So, was it the aircraft, or my own “plumbing” that made the difference?

That’s an interesting question! Both the new and old 737s are designed so that the cabin altitude during cruise flight doesn’t exceed about 8000 feet.

However, the pressurization system on the newer 737 is an improved digital system that includes features designed to increase comfort. The pressure changes, while still there, should be a little smoother and less noticeable, making the flight a little easier on your ears. As you noticed, passengers like it.

Very informative, thanks. I was just diagnosed w a tiny brain aneurysm and am nervous about flying next week although 2 neurologists said it was fine and a nurse told me that its a myth that you can’t fly. Someone else said your brain is protected ftom pressure bc it’s in a fluid sac and i read that it’s ok bc the cabin is pressurized which led me to this article. Do you have any insight about how flying affects the brain and blood vessels and if there are any dangers?

Hi Ken,
Many thanks for your explanation because I recently traveled on an Australian domestic flight on which I experienced pain in one ear, and I was wondering how and at what level the pressure was maintained.
I also noticed that perfumes seemed to suddenly be detected in the cabin air. Can and do pilots inject perfumes or other chemicals into the cabin air stream?
Thanks again

Aircraft pressurization systems generally maintain the cabin pressure altitude at about 7000-8000 feet or about 11.3 psi. If you have a sinus blockage, it doesn’t take much change in pressure to experience inner ear pain.

I’ve never heard of anyone injecting perfumes or chemicals into the cabin air stream. No aircraft that I am familiar with has that capability. Most likely, another passenger was putting on, or spraying perfume. You might also have been smelling a deodorizer in a lavatory (which is far better than smelling the lavatory!).

I think you are asking: “What happens when the pressurization system fails?”
If the system and its redundant, back up systems stop working, the outflow valve is designed to close which will slow (but not stop) the aircraft from depressurizing. In this situation, the flight crew will begin an emergency descent. At lower altitudes there will be sufficient atmospheric pressure for normal breathing.

Thanks Ken for your reply.. and here comes my real doubt… at the time of depressurisation, when the aircraft starts to descent to a lower altitude, how the cabin pressure will get equal to the outside pressure.. I mean fuselage is air tight right? Then how its done…?

It surprises many people that an aircraft fuselage is not airtight. Even with the outflow valve fully closed, air still leaks out of it. Window and door seals also leak a little bit of air. So, if the pressurized air source is interrupted, the fuselage will slowly lose pressure. This is why flight crews will immediately begin descending the aircraft if there is a serious pressurization problem.

Pressurization systems on every aircraft are a little different. The valves are usually operated by an electric motor that receives a signal from the controller. As you go through your training, you’ll learn the small details!

Unfortunately, the experiment of “fill a balloon and weigh it” won’t work the way that you describe. Of course, air *does* have mass… but filling the balloon also displaces atmospheric air… so the balloon “floats” in the atmosphere by exactly the same amount of additional weight imparted by the mass of the air inside it. So a balloon inflated with air will weigh the same on a scale as an unfilled balloon.

An easy way to think about this is to consider a helium-filled balloon on the surface of the earth. Helium has some mass, but it’s less dense than air. Helium balloons float because they displace more air-mass than the mass of the helium inside of them. So filling a helium balloon will make it apparently weigh less, even though you’ve increased the mass.

Decompression and “decreased air pressure” are the same thing: the reduction of air pressure in the cabin of an aircraft. After an aircraft takes off, the pressure inside the cabin decreases at a slower rate than the pressure outside the aircraft as it climbs.

Your posts are great and interesting for me and I always read it… I am Iranian and I translate your post to Persian for my channel in telegram…I learn so much information from your posts and thank you for writing these posts.

Hello Ken,
I have a billion questions coming more as a result of curiosity than anything else. Recently we had a little baby and wife and I were trying to fly but the airline told us baby needed to be at least a month old to fly but Google said otherwise, what is your proffessionl opinion on flying a week old baby?

Hello Ken,
Great stuff on pressurizing the cabin. Quick question. How does the system maintain a cabin oxygen concentration (21% at sea level) of cabin air at cruise of 35K feet When there is very little oxygen in the outside air. My guess this is where the compression of outside air to be vented through the cabin plays a role???

Don’t try to overthink the system, it’s very simple. The air in our atmosphere is 21% oxygen. This percentage is roughly the same at all altitudes that an aircraft operates. At sea level, air molecules (including oxygen molecules) are close together. At 35,000 feet, the air molecules are spread out (but the oxygen content is still 21%). The pressurization system simply takes the “thin” outside air and pumps it into the cabin with enough to force to push the air molecules close together so they’re breathable by us humans.

Hi Ken, Thanks for a really informative article that answered all the questions that went through my mind on a flight from Singapore to Perth yesterday. It’s interesting that you busted the myth planes recycle the air through the cabin which makes it easy to catch a cold on a plane.
My question is does using bleed off air increase fuel consumption? I was on a Royal Jordanian Tri-star some years ago and several passengers fainted (including me) and we were told that the pilots would turn down the air-con to save fuel. Does that make sense?

Thanks for the comments and questions! I’ll try to clarify the system a little bit…

Does bleed air increase fuel consumption?
YES! We are essentially “stealing” compressed air from the engines and that comes at the cost of burning a little extra fuel. The less bleed air we steal, the lower our fuel burn is. The amount of fuel is relatively small on any flight, but it all adds up at the end of the year.

“Recycled” air: Actually, most modern aircraft do, sort of, recycle air. A typical component of the air conditioning system is the recirculation (or recirc) fan. The fan takes air, usually from a lower compartment, and pumps it back into the air conditioning ducts. The fans increase overall cabin air circulation while lowering the airflow required from the pressurization system. This saves a little bit of fuel ($$). Don’t worry, even with the recirc fan, there is always fresh air being pumped into the cabin and stale air exiting the outflow valve. Even the most efficient aircraft pressurization system has more air turn-over than your home or office.

Your last comment has me stumped. Pilots really can’t “turn down” the pressurization. Some systems allow us to increase/decrease the flow, which affects the noise level and fuel burn slightly, but the aircraft will still maintain proper pressurization. If the cabin pressure is reduced to a dangerous level, the oxygen masks will drop down. I have no idea what could have been going on in your Tri-Star (one of my favorite classic jets!).

Hi Ken. I came across your article while trying to research whether or not I can ship an exterior steel patio door by way of airfreight on a major passenger airline without any Argon gas escaping. I was told by the airline, after they checked with their Dangerous Goods (DG) Manager, that it was okay because it was not compressed. I am still concerned that the gas may escape under pressure in the cargo area and the door would be rendered useless once it gets to its destination, because once the gas escapes, fog, or steam, builds up between the glass and appears cloudy. My concern arose when the shipping worker told this to me when he asked how I was shipping it. I’d appreciate any input on this concern. Thank you.

Came across this Article, while searching for Cabin Pressure Related Psychological / Physiological effects. During our Last flight with Family (My Wife’s Maiden Night Flight), she started feeling suffocated, when the cabin lights were dimmed, which could be attributed to Psychological effect, but soon (even before and during Take OFF) started reporting symptoms of Barotrauma (Pain in various parts of “gastrointestinal tract”), which could not be attributed to Psychological effort.

My Question here is:
1. Will the Cabin Pressure be altered even before Take OFF (During Engines are Revved up in full Throttle, in preparation of Take OFF in Few Seconds).

2. I came across another article (Link Given Below), which Claims that Human capacity to adapt to low pressures differ between day & Night. Is It TRUE!!!?

There are a lot of really bad (incorrect) responses in the Quora link. Hypoxia effects are the same on the body, day or night. At night, we may notice a difference in vision due to the way rods and cones in the retina work. Without oxygen, you’ll die in the same amount of time, day or night. 🙂

Most airliners pressurize the cabin slightly just before takeoff. This small amount of pressurization should theoretically cause gasses in the body to be reduced in size (take up less space in the digestive system and sinuses). If this small amount of pressurization has any effect on the body at all, it would reduce gas pains (the gas takes up less space). Once airborne, the cabin pressure will slowly begin to drop (cabin altitude climbs). The slow drop in cabin pressure causes bodily gasses to expand. That’s what usually causes gas pains in the digestive system and clogged sinus pain.

It’s difficult to say exactly what was going on with your wife. I hope her next flight goes better!

Was flying a King Air 200, very high pressure system across the region and PA/DA was extremely low.
Field Elev. 1000 (Cabin Controller setting)
DA -3,200 ft
PA -2,700 ft
Alt.Setting 30.66
Temperature -16C

During the descent the aircraft started to depressurize at 4,000ft MSL. Is this normal due to the environmental conditions? the Cabin Controller would only adjust to -1500ft.

That doesn’t sound like normal behavior for a pressurization system. However, I’m not familiar with the King Air. You should consult the manufacturer’s operating manual or your company’s training documentation for more information.

Hi Ken, a first class article. I train commercial divers in the UK and often use the analogy of pressurised aircraft to help understand that the partial pressure of oxygen is what sustains life rather than the percentage of oxygen in the mix. A “deep sea” diver living in saturation will survive breathing oxygen percentages of less than 5% at depth and this can be difficult to understand for many. Your article helps make it very clear for many in the way you present it. Forgive me if I borrow some of your knowledge to educate the commercial diving community. A fantastic article, and very well presented, many thanks.

I did have one question, however. For narrow body (737, a321) shorter duration flights <45 mins that fly at a lower altitude of 15-20k feet, would cabin pressure be higher than one flying at 35,000 feet for 2 hours? So, instead of a psi between 11-12, it would maybe be even higher at 12-13 and thus cabin altitude would be lower than 5-6k feet?

That’s exactly right. If the aircraft is cruising at a lower altitude, the pressurization system will provide a lower cabin altitude. A few nights ago we had a short hop on the 767. Our cruise altitude was 24,000. I think our cabin was about 2,000 feet.